(i) Field of the Invention
The present invention relates to a perpendicular magnetic recording medium which is suitable for high-density magnetic recording and a magnetic recording apparatus using the same.
(ii) Description of the Related Art
A magnetic disk storage device that has been actually used at present utilizes longitudinal magnetic recording. It is the technical problem to form longitudinal magnetic domains at high densities on a longitudinal magnetic recording medium which is easily magnetized in a direction parallel to a disc substrate, parallel to the surface of the disk substrate. To increase an areal recording density, particularly a linear recording density, in this recording mode, it is required to reduce the thickness of a magnetic film for recording while improving the coercivity of the longitudinal magnetic recording medium. When the coercivity is greater than 4 kOe, it becomes difficult to conduct recording by a magnetic head. Meanwhile, when the thickness of the magnetic film made of, for example, a Co alloy is equal to or smaller than 15 nm, intensity of recorded magnetization decreases with time due to thermal fluctuation. The longitudinal magnetic recording essentially has the problem that a magnetization transition region having wide boundaries is formed due to opposing magnetizations of adjacent recording bits. Therefore, mainly due to the above reasons, a technical difficulty is expected in order to achieve an areal recording density of 40 Gb/in2 or higher.
The perpendicular magnetic recording, in which magnetization occurs in a direction perpendicular to the surface of a thin-film medium, differs from the conventional longitudinal magnetic recording in its recording principle and mechanism for causing a medium noise. Since adjacent magnetizations are antiparallel in the perpendicular magnetic recording, it has drawn attention as a recording mode that is essentially suitable for high-density magnetic recording, and a medium structure suitable for the perpendicular magnetic recording has been proposed. The perpendicular magnetic recording is classified into two types, one of which uses a single-layer perpendicular magnetization film and the other of which uses a perpendicular magnetization film having a magnetic back film formed thereon. The technique using a dual-layer perpendicular magnetic recording medium using the magnetic back film is described in, for example, IEEE Transaction on Magnetics, Vol.MAG-20, No.5, September 1984, pp.657-662, xe2x80x9cPerpendicular Magnetic Recordingxe2x80x94Evolution and Futurexe2x80x9d. As the perpendicular magnetic recording medium for this recording mode, there has been considered a medium having a perpendicular magnetization film made of a Coxe2x80x94Cr alloy formed on a soft magnetic back film made of a Permalloy.
To commercialize a magnetic recording apparatus capable of high-density magnetic recording of 40 Gb/in2 or higher by the perpendicular magnetic recording using the dual-layer perpendicular magnetic recording medium, it is essential to reduce the medium noise, secure a magnetic signal strength from recorded magnetization and improve the recording efficiency of a recording head.
The medium noise is manufactured from both the perpendicular magnetization film and the magnetic back film, and the spike noise manufactured from the magnetic back film has been particularly problematic. An example of such a noise is described in, for example, IEEE Transaction on Magnetics, Vol.MAG-20, No.5, September 1984, pp.663-668, xe2x80x9cCrucial Points in Perpendicular Recordingxe2x80x9d. To deal with such a problem, a method of forming a longitudinal magnetization film underneath the magnetic back film has been proposed in, for example, The Magnetics Society of Japan Journal, Vol.21, Supplement No.S1, pp.104-108, xe2x80x9cImprovement in S/N of three-layer perpendicular medium and stability of recording signalxe2x80x9d. Such proposals have not been always satisfactory for commercializing a magnetic recording apparatus capable of high-density magnetic recording of 40 Gb/in2 or higher.
As for securing the magnetic signal strength from recorded magnetization, although the dual-layer perpendicular magnetic recording medium can secure almost twice as much signal strength as the single-layer perpendicular magnetic recording medium having no soft magnetic back layer, it has had a problem with the spike noise which is inherent in the soft magnetic back layer as described above. In a magnetic recording system comprising the dual-layer perpendicular recording medium and a single pole-type recording head, it is necessary for improving the recording efficiency of the recording head to urge the quick regression of a magnetic flux, which has emerged from the recording pole, to the head after passing through the perpendicular magnetization film. For this reason, the soft magnetic back film must be at least several times thicker than the perpendicular magnetization film for recording.
It is the object of the present invention to provide a perpendicular magnetic recording medium for achieving a high-speed and high-density recording density of 40 Gb/in2 or higher and to facilitate the attainment of a high-density recording and reproducing apparatus, by securing (1) a high-density magnetic recording property, (2) the signal strength from recorded magnetization, and (3) the efficiency of the recording head, which are the characteristics of the magnetic recording system comprising the dual-layer perpendicular magnetic recording medium and the single pole-type recording head, and providing a method for preventing the. production of the noise inherent in the magnetic back layer, which has been a big problem heretofore.
To attain a perpendicular magnetic recording medium having a low-noise property, high recording efficiency of a recording head and a high signal output property from a recording bit, the present invention is constituted by a perpendicular magnetic recording medium having a perpendicular magnetization film formed on a non-magnetic substrate via a magnetic back film, in which the magnetic back film comprises two or more soft magnetic films which are separated at least by a non-magnetic layer, the soft magnetic film closer to the perpendicular magnetization film serves as a soft magnetic keeper layer for keeping perpendicular magnetization, and the magnetization of the soft magnetic film(s) closer to the substrate has magnetization orientation(s) different from the above soft magnetic keeper layer.
The magnetic back film in the dual-layer perpendicular magnetic recording medium serves to (1) increase the intensity of magnetization leaked from the surface of the medium while stabilizing the magnetization recorded on the perpendicular magnetization film and (2) increase the recording efficiency of the recording head. The present inventor has found according to experiments and studies that the conventional problems can be solved with the above features intact by multiplying the soft magnetic back layer in a certain multilayer structure.
A description will be given to the structure and effect of the perpendicular magnetic recording medium according to the present invention with reference to FIGS. 1 and 2. FIG. 1 is a cross-sectional schematic diagram of the perpendicular magnetic recording medium according to the present invention, and FIG. 2 exemplarily shows the magnetization orientations in the soft magnetic film at the Axe2x80x94A cross-section and the Bxe2x80x94B cross-section. In the present invention, as the fundamental structure of the soft magnetic back film, a structure is employed that comprises a soft magnetic film 17, which serves to increase the intensity of magnetization leaked from the surface of the medium while stabilizing the magnetization recorded on the perpendicular magnetization film, soft magnetic films 13 and 15 supplied for particularly improving the recording efficiency of the recording head, and a non-magnetic layer 16 which is interposed between the film 17 and the films 13 and 15. FIG. 1 shows the structure having two soft magnetic films of the latter type.
It is known that the spike noise inherent in the dual-layer perpendicular recording medium is manufactured with regard to the magnetic domain boundary generated in the soft magnetic back film. In the present invention, as shown in FIG. 2, the magnetization orientations 20 and 21 of the soft magnetic films 13 and 15 which take up the main portion of the soft magnetic back film are made antiparallel to each other, and when the substrate 11 is in the form of a disk, the magnetization orientations can be aligned parallel to the circumferential direction of the disk substrate. By setting the magnetization orientations in the circumferential direction, the generation of the magnetic domain boundary causing the noise can be suppressed. Further, as shown in FIG. 1, the soft magnetic films 13 and 15 adjacent to each other via a non-magnetic layer 14 are characterized in that they are apt to be magnetically coupled such that they are antiparallel to each other in terms of magnetic energy. Even when two or more of the magnetic films are formed, the above antiparallel relationship is apt to be established. It is desirable that an adhesion-reinforcing layer 12 be generally formed between the substrate 11 and the first soft magnetic film 13. By forming a plurality of the layers 12 and incorporating anti-ferromagnetic films and ferromagnetic films for fixing the magnetization orientations of the soft magnetic films into the plurality of the layers, a more desirable practical effect can be obtained.
To define the magnetization orientation of the soft magnetic film in the circumferential direction of the disk substrate, a magnetic field that spins in the circumferential direction of the disk substrate may be applied during or after the formation process of the thin-film. This is achieved by utilizing the phenomenon which is caused by placing an electric conductive wire in such a position that it passes through the hole made in the central portion of the disk at a right angle and generating a magnetic field in the form of a concentric circle around the wire by passing an electric current through the wire.
Further, on these soft magnetic films 13 and 15, a soft magnetic layer 17 which serves to increase the intensity of magnetization leaked from the surface of the medium while stabilizing the magnetization recorded on a perpendicular magnetization film 18 is formed via the non-magnetic film 16. This soft magnetic film 17 serves not only to increase the recording efficiency of the recording head as does the above soft magnetic film in magnetic recording by the recording head but also to stabilize recorded magnetization by forming a closed magnetic path corresponding to the state of the recorded magnetization underneath the magnetic domains formed in the perpendicular magnetization film 18 as shown in FIG. 1. This soft magnetic film 17 amplifies the intensity of magnetization from the surface of the medium by forming magnetically continuous horseshoe-shaped magnets by adjacent magnetic domains formed in the perpendicular magnetization film 18 underneath the domains.
The soft magnetic film 17 which serves as described above is not necessarily as thick as the conventionally known soft magnetic film. Further, a film for controlling the crystal growth of the perpendicular magnetization film may be formed between the soft magnetic film 17 and the perpendicular magnetization film 18. However, to take advantage of the characteristics of the dual-layer perpendicular magnetization film, care must be taken such as to ensure that the thickness of the film for controlling the crystal growth should be set to be thinner than the shortest bit length in magnetic recording. Further, in order not to deteriorate the magnetic recording property, this film is preferably made of a weak magnetic material having a saturation magnetization of not higher than 50 emu/cc or non-magnetic material.
According to the experiments and studies made by the present inventor, it has been found that the condition which amplifies the intensity of magnetization from the surface of the medium but does not allow the spike noise to be noticeable depends on the linear recoding density of magnetic recording, when the saturation magnetization and thickness of the soft magnetic film formed underneath the perpendicular magnetic film are defined as Bsm and t, respectively. When the shortest bit length in magnetic recording and the average saturation magnetization of the perpendicular magnetic film are defined as Bmin and Ms, respectively, 0.5 Bminxc2x7Msxe2x89xa6Bsmxe2x88x92t must be satisfied. For example, when the maximum linear recording density is defined as 500 kFCI (kilo Flux Change per. Inch), the average saturation magnetization of the perpendicular magnetic film as 0.4 T and the saturation magnetization of the soft magnetic film as 1 T, its thickness, t, satisfies 10 nmxe2x89xa6t. When Bsmxe2x88x92t becomes smaller than 0.5 Bminxc2x7Ms, the above effect becomes weak, and the intensity of magnetization from the surface of the medium lowers to a value which is almost the same as that when the single-layer perpendicular recording medium is used. Further, the maximum value of t does not so much depend on the recording density and the magnitude of the saturation magnetization of the soft magnetic film, and when it is higher than or equal to 100 nm, the thickness of the soft magnetic film increases, and the magnetic domains irrelevant to the information on the perpendicular magnetization supplied to the soft magnetic film are liable to be formed and become the sources of the spike noises. Further, a soft magnetic material having a larger saturation magnetization Bsm is preferably used in order to bring about the effect of output amplification by reducing the thickness of the film. It is effective to use a material having larger saturation magnetization than that of the soft magnetic film(s) that serves to promote the recording efficiency of the head mounted on the substrate side.
Further, when the thickness and saturation magnetization of the m-th soft magnetic film including the soft magnetic films formed in the vicinity of the perpendicular magnetization film is defined as Tm and Bsm, respectively, and the saturation magnetization (Bsh) and track width (Tw) of the magnetic pole material for the recording head are considered, it is desirable that 0.16Bsh xe2x88x92Twxe2x89xa6xcexa3(Bsmxe2x88x92Tm)be satisfied. When 0.16Bshxe2x88x92Tw greater than xcexa3(Bsmxe2x88x92Tm), there occur such problems that the recording efficiency of the recording head lowers and that demagnetization in recording becomes noticeable. Further, although the recording efficiency improves as the total thickness of the soft magnetic films increases, the increase in the film thickness is accompanied by an increase in the degree of roughness on the medium surface or the like. Therefore, it is desirable that xcexa3(Bsmxe2x88x92Tm)xe2x89xa6Bshxe2x88x92Tw be satisfied.
As the perpendicular magnetization film used in the present invention, any conventionally known types of perpendicular magnetization films can be used. That is, as the perpendicular magnetization film, there can be used a polycrystal film made of a Co alloy, a Coxe2x80x94Pt alloy and an Fexe2x80x94Pt alloy, a polycrystal multilayer film made of a Coxe2x80x94Co alloy an d a Ptxe2x80x94Pt alloy, a polycrystal multilayer film made of a Coxe2x80x94Co alloy and a Pdxe2x80x94Pd alloy, or the like. Further, a perpendicular magnetization film comprising an amorphous film containing rare earth elements can also be used.
As the soft magnetic material, there can be used Fe group-based alloys such as Fexe2x80x94Ni, Fexe2x80x94Si, Fexe2x80x94Al, Fexe2x80x94Alxe2x80x94Si and Fexe2x80x94Cr alloys, Ni group-based alloys such as Nixe2x80x94Fe and Nixe2x80x94Mn alloys, Co group-based alloys such as Coxe2x80x94Nb, Coxe2x80x94Zr and Coxe2x80x94Fe alloys, or a soft ferrite represented by MOxc2x7Fe3O4(M=Fe, Mn, Ni, Co, Mg, Zn or Cd). Particularly, as the soft magnetic film formed in the vicinity of the perpendicular magnetization film, there can be suitably used Fe group-based alloys such as Fexe2x80x94Taxe2x80x94C, Fexe2x80x94Sixe2x80x94Al, Fexe2x80x94Coxe2x80x94C, Fexe2x80x94Sixe2x80x94B, Fexe2x80x94Bxe2x80x94C and Fexe2x80x94Bxe2x80x94Cxe2x80x94Si alloys, and Co group-based alloys such as Coxe2x80x94Nbxe2x80x94Zr, Coxe2x80x94Moxe2x80x94Zr, Coxe2x80x94Taxe2x80x94Zr, Coxe2x80x94Wxe2x80x94Zr, Coxe2x80x94Nbxe2x80x94Hf, Coxe2x80x94Moxe2x80x94Hf, Coxe2x80x94Taxe2x80x94Hf and Coxe2x80x94Wxe2x80x94Hf alloys, all of which are capable of forming a soft magnetic film that has a high saturation magnetization of not lower than 1 T and is amorphous or microcrystalline. When this material is amorphous or microcrystalline, the crystal grains of the perpendicular magnetization film formed thereon are also liable to become minute, and such a material is suitable for imparting high perpendicular magnetic anisotropy.
As the non-magnetic material interposed between the soft magnetic films, there can be used an element selected from the group consisting of B, C, Mg, Al, Si, Ti, V, Cr, Cu, Zr, Nb, Mo, Ru, Hf, Ta, w and Au, an alloy comprising these elements as main components, a compound selected from the group consisting of Si3N4, BN, B4C, NiO, Al2O3, SiO2, CaO, ZrO2 and MgO, or a mixed crystal comprising these compounds. To improve the high-frequency recording property of magnetic recording, a non-magnetic material having high electric resistance, that is, a material selected from the group consisting of B, C, Si, Si3N4, BN, B4C, NiO, Al2O3, SiO2 and CaO or a mixed crystal material comprising any of these materials as main components, is suitably used.